Conductive pedestal on pad for leadless chip carrier (LCC) standoff

ABSTRACT

A device and method for insuring the separation between a leadless chip carrier and printed wiring board, comprising aligning and attaching conductive pedestals to contact pads of either member and embedding the pedestals into the solder columns which are used to provide electrical connection. The conductive pedestals are comprised of an electrically conducting metal, solder, alloy or composite which will also provide thermal dissipation in selected designs.

This application is a divisional of application Ser. No. 09/229,622,filed Jan. 13, 1999 now U.S. Pat. No. 6,472,611, and claims priorityfrom provisional patent application No. 60/073,667, filed Feb. 4, 1998.

FIELD OF THE INVENTION

This invention relates to an improved method of surface mounting anintegrated circuit package, more specifically a leadless chip carrier,to a printed wiring board.

BRIEF DESCRIPTION OF THE PRIOR ART

This invention relates to an improvement in the surface mounting of anintegrated circuit package, more specifically a leadless chip carrier,to a printed wiring board.

A leadless chip carrier is an integrated circuit package which includesa ceramic substrate on which there is provided a pattern of contactpads. A corresponding pattern of contact pads is provided on the printedwiring board. When the leadless chip carrier is mounted to the printedwiring board, the contact pads are electrically interconnected.

Various approaches to the mounting and electrical connection of theleadless chip carrier to printed wiring board have been proposed andimplemented in the past. All of the known approaches are disadvantageousfor one or more reasons. One approach is to provide a connector socketfor receiving the leadless chip carrier on the printed wiring board.This approach is relatively expensive.

Another mounting approach is the use of a Chip Carrier Mounting Device(a trademark of Raychem Corp.) which is an array of high temperaturesolder wire leads having an embedded helical copper braid. The array isheld in place by an dissolvable carrier which is temporary; the carrierbeing used to facilitate alignment of the wire leads to the contact padsof the leadless chip carrier The carrier is removed after the leads aresoldered to the leadless clip carrier. Disadvantages of this approachinclude the use of the temporary carrier which has a limited shelf lifeand is adversely affected by humidity. Additionally, this approachrequires custom design which adds to the expense.

Another approach is the use of edge clips which are created from metalstampings and are available on continuous reels. The edge clips areintended to clip onto the edge of the leadless chip carrier using aspring retention mechanism. Some disadvantages are that they do not fitmany leadless chip carrier package and they are frequently difficult toassemble to leadless chip carriers. Additionally, the use of edge clipsresults in excessively high stand off of the leadless chip carriers fromthe printed wiring board which reduces the packing density.

Yet another approach is the use of a lead wire array which providespre-leading of a leadless chip carrier for subsequent solder assembly toa printed wiring board. The lead wire assembly is formed from a lengthof bare wire which is first bent into a planar serpentine shape. Thebending results in a plurality of parallel segments which correspond tospacing of the contact pads. Portions of the parallel segments areflattened in order to provide a larger contact area for solderconnection at the contact pads. The parallel segments in the first planeare then attached to the respective contact pads on the leadless chipcarrier and the parallel segments in the second plane are attached torespective contact pads on the printed wiring board. The reversingsegments of the wire are removed to eliminate short circuits between thecontact pads. Some disadvantages of this technique are that it is laborintensive, results in an assembly that is difficult to clean and standoff devices are required to prevent collapse onto the printed wiringboard.

The most widely used approach is to directly solder the contact pads ofthe leadless chip carrier to the contact pads of the printed wiringboard. Since the leadless chip carriers have a ceramic substrate, theyhave a lower coefficient of thermal expansion than printed wiring boardswhich are typically used in the industry. Therefore unless the thermalexpansion coefficient of the printed wiring board matches that of theleadless chip carrier, a reliability problem ensues as a result ofexcessive thermal-mechanical stresses placed on the solder joints. Theuse of component stand off devices to secure a separation of definedheight has been shown to minimize the problem of thermal mismatch. Thistechnique is used routinely. In this technique, solder masks are used tobuild stand off devices on the printed wiring board. Typically one standoff device per side of the leadless chip carrier is required and theymust adhere to strict requirements for height, as well as for spacingbetween the stand off devices and contact connections. Fabrication ofsaid solder mask devices is both time consuming and expensive.

SUMMARY OF THE INVENTION

It is the object of this present invention to provide an approach tocontrol the separation between leadless chip carriers and printed wiringboards by forming stand off devices. Said devices are provided bymounting conductive pedestals within the contact pad areas. Thisapproach is economical and does no have the disadvantages of thetechniques described above.

In accordance with the present invention, there is provided a reliable,flexible and low cost method of assembly to control the separationbetween leadless chip carriers and printed wiring boards. This approachis used in conjunction with directly soldering the contact pads of theleadless chip carriers to the contact pads of the printed wiring boards.Such a separation is necessary to avoid overstressing the solder jointswhich provide electrical and mechanical connection between the twomembers. Further, this separation supports cleaning of fluxes or othercontaminants which may deteriorate reliability and/or support formationof short circuits between contact areas.

The foregoing and additional objects are attained by providingconductive pedestals within the solder pad area. The conductive pedestalcomprises an electrically conductive material of controlled dimensionswhich set the stand off height. Further, the conductive pedestals retainstandoff dimensions within the specification during and subsequent tothe solder reflow process. Volume of the conductive pedestal is smallwith respect to that of the solder column which connects the chipcarrier and printed circuit board and does not significantly alter theproperties of said connectors. Said pedestal is compatible with thesolder used for connecting the contacts of the leadless chip carrier andthose of the printed wiring board.

The conductive pedestals are attached to contact pads of either theleadless chip carriers or to those of the printed wiring boards prior toembedding in the connecting solder columns.

In order to maintain the required coplanarity between chip carrier andboard, conductive pedestal size is controlled, and the pedestals must belocated so that the package will be maintained parallel to the printedwiring board; typically at least one conductive pedestal is necessary oneach side of the package. Additional pedestals provide redundancy whichcan further insure control of the stand off height coplanarity.

Further, because the conductive pedestals are contained within the areaof the solder column, they add no interference for cleaning between theconnectors, but they do aid in the cleaning efficiency as a result ofthe standoff provided between chip carrier and board:

Solders, alloys, metals or composites which are conductive areacceptable materials for conductive pedestals, so long as they arecompatible with eutectic solder and maintain dimensional stabilityduring solder reflow.

The invention herein comprises a method to transfer conductive pedestalsto the contact pads. The precise location of the small pedestals on thecontact pads is not critical. One method which is readily automated,high speed, low cost and is compatible with current manufacturingtechniques provides for transfer of preformed spheres.

The spheres when attached to contact pads form conductive pedestals.This method comprises forming an array of patterned areas which registerto a location for conductive pedestals. One conductive pedestal sphereis captured per area and retained until the spheres are aligned to thereceiving pads. The preferred method for forming the patterned arraysprovides a photoimagable adhesive coated on a transparent carrier film.One metal or alloy sphere is captured in each tacky areas and retaineduntil the spheres are aligned on the receiving contact pads. Uponheating, the adhesive loses tackiness and the pedestals are affixed tothe receiving pad by techniques such as reflowing of solder, alloy orcomposites such as polymers filled with metal.

A suitable film with photosensitive adhesive to form tacky patternedareas is available from E. I. duPont de Nemours & Company. The use ofsuch a transfer method has been disclosed previously in U.S. Pat. No.5,356,751 and is incorporated herein by reference. The specificapplication of a small conductive standoff within a much largerconductor area for the purpose of providing standoff is novel.

Alternate technique for transfer of such spheres has been demonstrated,such as by patterning an array of dimples in an film which will not wetto solder, capturing a sphere of solder in each dimple, aligning thepattern to the contact pads and reflowing onto the contact pads. Stillother techniques may be used to align and attach preformed conductivepedestals to the conductor pads.

Conductive pedestals attached to either the leadless chip carrier or theprinted wiring board do not alter the normal manufacturing procedure fordirect solder connection. Contacts on the printed wiring board arecovered with solder paste, the component aligned and placed byconventional techniques and the solder is reflowed by the existingmanufacturing procedure.

A further application of the conductive pedestal stand off devicecomprises populating thermal pads of leadless chip carriers in the sameoperation as forming conductive pedestals. Thermal pads are notelectrically connected; they are typically located in the center of thepackage, directly under the integrated circuit and are provided in anattempt to dissipate heat into the printed wiring board. Owing to thesmall size and placement accuracy provided, pedestals which support heatdissipation are readily forming using the same devices, methods and atthe same time as those for conductive pedestal stand off devices.

The use of conductive pedestals embedded within solder columns whichconnect leadless chip carriers to printed wiring boards differs fromball grid array connections in that (1) only one pedestal per packageside is required, (2) pedestals are not the primary electrical ormechanical connection, but instead becomes a part of said connection and(3) placement of said pedestals is not critical as long as they arewithin the contact pad areas. Further, conductive pedestals embeddedwithin solder columns differs from Chip Carrier Mounting Devices for thesame reasons as those for ball grid arrays, but also a less complex andmore reliable assembly process is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1A is a perspective view of a leadless chip carrier showing typicalpattern of contact leads.

FIG. 1B is a perspective view of a portion of a printed wiring boardshowing corresponding pattern of contact pads.

FIG. 1C is perspective view of a portion of a printed wiring board withcontact pads and solder mask stand off devices.

FIG. 2 illustrates a cross sectional view of a leadless chip carrierwith contact pads directly soldered to the contact pads of a printedwiring board.

FIG. 3 illustrates a cross sectional view of a leadless chip carrierwith conductive pedestal stand off devices directly solder to thecontact pads of a printed wiring board.

FIGS. 4A-C illustrate cross sectional views of leadless chip carrierwith conductive pedestals attached before and after attachment to aprinted wiring board.

FIG. 5A depicts a conductive pedestal which was attached to a leadlesschip carrier Vs 5B in which the pedestal was attached first to a printedwiring board.

FIG. 6 illustrates the noncriticality of conductive pedestal placementwithin the pad area

FIG. 7 depicts some options for population of contact pads withconductive pedestals

FIG. 8 depicts placement of conductive pedestals on alternate shape chipcarrier packages.

FIG. 9 illustrates the processing steps for attachment of conductivespheres by the tacky dot transfer method.

FIG. 10 illustrates a leadless chip carrier with thermal pads.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, FIG. 1A shows a typical leadless chip carrier100 which includes a ceramic substrate with a major surface 101. On thesurface 101 is a pattern of contact pads 110 which typically extendalong the edges of the surface 101. FIG. 1B shows a portion of a printedwiring board 120 to which the leadless chip carrier 100 is to bemounted. The printed wiring board 120 has a major surface 121 on whichthere is a pattern of contact pads 130 corresponding to the pattern ofcontact pads 110.

FIG. 2 illustrates a cross sectional view of the direct soldering ofcontact pads 110 of the leadless chip carrier 100 to the contact pads130 of the printed wiring board 120. Because the printed wiring board120 has a much higher coefficient of thermal expansion than that of theceramic leadless chip carrier 100, thermally induced mechanical stressesare placed on the solder joints which may result in failures. Suchstresses can be minimized by making the circuit board 120 and chipcarrier 100 of similar materials. However, this would make a veryexpensive circuit board if the board were to be made of ceramic. On theother hand, it the leadless chip carrier package were made of circuitboard composition, it would negate the choice for a reliable ceramicpackage.

In actual practice, the fatigue life of solder joints has been enhancedby increasing the height of the solder columns 116 which connect lifeleadless chip carrier 100 and printed circuit board 120. The most widelyused approach for insuring controlled height of the solder columns 116has been to form stand off devices 131 on the printed wiring hoard asshown in FIG. 1C. One common technique for forming the stand off devices131 is by patterning and etching solder mask material on the printedwiring board 120. The solder mask stand off process is both timeconsuming and expensive. Chemical usage and waste disposal add to theexpense of this labor intensive process. Typically the solder mask standoff devices 131 are 0.007 to 0.010 inches in height and are formed intwo steps, each step adding approximately 0.004 inches and requiring 2to 4 hours per step. Typically there is one such device is formed at ornear the corner of each contact pattern array 110 as is illustrated inFIG. 1C. Separation between leadless chip carrier 100 and printed wiringboard 120 is required not only to minimize solder joint fatigue, butalso to facilitate cleaning flux and other contaminants. It can be seenfrom the illustration of solder mask stand off devices 131 that thesedevices do occupy space on the board and can inhibit the flow ofcleaning solutions.

According to this invention, a stand off device is provided asillustrated in FIG. 3, wherein a conductive pedestal 301 is embedded inthe solder column 116. Said conductive pedestal 301 serves to controlstand off between the leadless chip carrier 100 and printed wiring board120, and because it is contained within the area of the solder columnitself, it does not occupy additional space on the board and thereforedoes not interfere with cleaning processes.

To further explain the conductive pedestal stand off device 301 and itsuse, some requirements on the leadless chip carrier should beunderstood. The contact pads 110 of leadless chip carrier devices arerelatively large in order to assure good contact; i.e., typically thepads are in the range of 0.025 by 0.050 inches. The minimum separationrequirement between carrier 100 and board 120 is 0.008+0 002/−0.001inches from bare laminate board; however, reliability improvement hasbeen shown if the height of the solder column 116 is controlled at thehigh end of that specification. The maximum separation is defined by thestencil used to screen solder paste on the printed wiring board contactpads 130

A conductive pedestal 301 comprises an electrically conductive materialof controlled height to meet this specification and which is compatiblewith the eutectic solder of the solder column 116. Spheres of conductivemetals or alloys are readily available with controlled diameter of 0.008to 0.010 inches and which can be attached to contact pads to formconductive pedestals. Attachment techniques are by solder or alloyreflow, or by conductive adhesives. The conductive pedestals 301 aresubsequently embedded within the solder column 116 to form stand offdevices. Several metals, composites and alloys or solders meet theserequirements

In the preferred embodiment, the conductive pedestal 301 is formed from10 Sn/90 Pb solder spheres which reflow at a higher temperature than theeutectic 37 Sn/37 Pb used in the solder column 116. Liquidus temperatureof 10 Sn/90 Pb is near 300 deg C., whereas that of 37 Sn/63 Pb is 183deg C. Attachment temperature of the higher melting 10 Sn/90 Pb solderin controlled so that a bond to the contact pad is accomplished, but thesphere retains approximately eighty percent of its height; i.e. thesphere with starting diameter of 0.010 inches will provide anapproximate stand off height of 0.008 inches. Further, due to the highermelting temperature of the 10 Sn/90 Pb solder, it maintains the standoff height requirement during reflow of the eutectic solder to theprinted wiring board.

The solders are completely compatible so that no separation between thematerials is created by dewet during assembly processing. The volume ofconductive pedestal 301 is small in comparison with that of the soldercolumn 116 so that it does not significantly alter the electrical,thermal or mechanical properties of said connecting column. In thepreferred embodiment, 10 Sn/90 Pb solder spheres 0.008 to 0.010 inchesin diameter form the conductive pedestals which are embedded in eutecticsolder columns of typical dimensions 0.025 by 0.050 by 0.009 inches. Thematerial choice is not limited to this selection

The simple design constraints for location of conductive pedestals aredepicted in FIGS. 5A, 5B, 6 and 7. It can be seen from FIGS. 5A and 5Bthat the conductive pedestal 301 can be formed by attaching spheres toeither the leadless chip carrier 100 as shown in FIG. 5A or to theprinted wiring board 120 as shown in FIG. 5B. Shape of the fillet at thecontact pad is the detectable difference, but there is no effect on thefinished device. If the conductive pedestal is attached to the leadlesschip carrier, the attachment process is carried out after electricaltesting of the device in order to avoid any change in the expensive testfixturing. Simple visual inspection for pedestal presence is the onlyrequirement. Alternately the conductive pedestals 301 can be attached tothe printed wiring board 120 as shown in FIG. 5B. Potential advantagesto this option are that the pedestals for more than one leadless chipcarrier can be processed in a single operation, and this would beparticularly advantageous for high volume production circuits. Thischoice will remain an option for the user.

From FIG. 6 it can be seen that precise placement of conductive pedestalis not critical. The contact pad 110 or 130 is large with respect to theconductive pedestal 301 and as long as the pedestal is within that padarea, the necessary functions of height separation by conductivepedestal is fulfilled. This feature allows latitude in the manufactureof said devices.

FIG. 7A shows that conductive pedestals can be placed on a single padper side of the contact pads 110 or 130, whereas FIG. 7B shows pedestalsfully populating each contact pad, and FIG. 7C shows an array ofpedestals with one redundant pedestal per side of the pads. Thepreferred embodiment provides redundancy, as in FIG. 7C, but avoidsexpectation that the pedestals form the primary means of electricalconnection. However, any combination of these options is acceptable solong as the minimum number and location of pedestals are provided tomeet the coplanarity requirements of the device.

Continuing with the design options for conductive pedestal stand offdevices, it can be seen from FIGS. 7 and 8 that different shapes ofleadless chip carrier packages 100 will logically require that thelayout of the stand off pedestals be modified; i.e., the rule of aminimum of one pedestal 301 per side would not be acceptable to thepackage with leads on two sides only as in FIG. 8A, but instead that twoor more pedestals must be placed on the long side. Alternately, on verysmall leadless chip carriers 100, as in FIG. 8B, coplanarity can besatisfied by pedestals 301 on three sides. Again, these options will bemanaged by the user and those versed in the art will understand and willfind the latitude of this design beneficial.

In summary design constraints for conductive pedestal stand off devicesare as follows: a conductive material which is compatible with solder ofcontrolled size which will be maintain height during reflow processing,and of small volume so as not to alter the properties of the soldercolumn connection. Location of the pedestals within the contact pad isnot critical, attachment can be to either the leadless chip carrier orprinted wiring board and the number of columns required is determined bycoplanarity.

Turning now to the method for forming conductive pedestal stand offdevices 301. A reliable, flexible, low cost process which is compatiblewith high volume manufacturing is provided for aligning and attachingconductive pedestals to the selected contact pads 110 or 130. Theprocess provides for forming an array of patterned areas which registerto the locations for conductive pedestals. One conductive pedestal iscaptured per area and retained until they are aligned to the receivingcontact pads 110 or 130. The preferred method for forming the patternedarrays and attaching to the contact pads is depicted by the process flowin FIG. 9. In step 1 there is provided a photopolymer adhesive coatingon a 0.003 inch thick polyimide film. In step 2 of FIG. 9, an array oftacky or sticky areas is formed in the adhesive by placing a phototoolover the coating and then exposing the coating to a dose of ultravioletradiation. Those areas protected by the phototool remain tacky whilethose areas exposed to the radiation lose their adhesiveness. The tackyareas formed register to the pattern of contact pads selected forconductive pedestals. A Mylar cover sheet is removed and an excess ofspheres which will form the conductive pedestals are loaded onto thefilm. One sphere is retained by each of the sticky areas as in step 3.In the preferred embodiment, 10 Sn/90 Pb solder spheres are 0.008 to0.10 inches in diameter. Excessive and unwanted spheres are removedmechanically in step 4. The populated film is now ready to be aligned tothe contact pads 110 or 130.

Fluxing is commonly used with solder reflow processing in order toprovide clean surfaces which will readily be wet by solder. In suchcase, flux is applied to either the spheres or to the contact padsurfaces as in step 5. The spheres are aligned over the contact pads asshown in step 6 before the film is lowered. Upon heating or exposure toultraviolet light the adhesive loses tackiness and in the preferredembodiment, solder reflow releases the spheres from the film andmetallurgically attaches them to the receiving contact pads 110 or 130as in step 7. In step 8, the polyimide film is removed from theassembly. These steps complete attaching a conductive pedestals toselected contact pads of either the leadless chip carrier or printedwiring board.

The next steps in forming conductive pedestal stand off devices isprovided in the assembly of leadless chip carriers to a printed wiringboard. In these steps the conductive pedestals, previously attached tosaid contact pads are embedded in the solder columns which connect thetwo circuits electrically and mechanically. All contact pads of theleadless chip carrier are directly soldered to those of the printedwiring board. Typically this process involves applying a solder paste toall contact pads of the printed writing board through apertures in astencil. The leadless chip carrier components are aligned and placedwith automated pick and place equipment. Contact pads with conductivepedestals are processed in precisely the same manner as those with nopedestals and the processing equipment requires no changes. Solderreflow is carried out using conventional convection or infrared furnaceswith predefined ramp rates and temperatures to accomplish reflow andcooling. The existence of conductive pedestals on some contact pads doesnot alter this process.

Conductive pedestal devices of the preferred embodiment were been testedon 28 pin leadless chip carriers and compared to use of stand off formedby solder mask processing. Test conditions were temperature cycling from−55 to 125 deg C. with 30 minute dwell at temperature. Solder jointintegrity was monitored by an event detector and the tests carried outto 50% failure. Failure mechanisms of the solder joints were analyzed bycross sectioning and inspecting by scanning electron microscope. Theresults found that the conductive pedestal devices were statistically asreliable as those formed by solder mask pedestals and no difference infailure mechanisms were detected. Cycle time reduction shows a 70%improvement for the conductive pedestal process, even in a developmentstage as compared to the solder mask pedestal technique in a maturestage.

A further embodiment comprises populating thermal pads of leadless chipcarriers by the same methods as those described forming the conductivepedestals. Further, thermal pads can be connected during the sameoperation as the conductive pedestals, but it is also possible to usethis technique for heat dissipation without the conductive pedestalstand off devices. Thermal pads are not electrically connected, butt areprovided in an attempt to dissipate heat from the leadless chip carrierinto the printed wiring board. Thermal pads frequently are provided inthe central area of the package, as is shown in FIG. 10. Owing to thesmall size and placement accuracy provided by the process detailed forconductive pedestal formation, thermal pads sill be populated withthermally conductive spheres. Many materials, including the 90 Sn/10 Pbsolder of the preferred embodiment provide good thermal conductivity andare acceptable choices. Similarly, connection to the board is preferablyby soldering with eutectic solder, as with the stand off devices, butany thermally conductive material is acceptable to provide heatdissipation because an electrical connection is not required.

What is claimed is:
 1. A method of attaching an integrated circuitdevice to a printed wiring board comprising the steps at forming tackyareas on a film to correspond to at least some of a plurality of contactpads on said integrated circuit device; loading conductive pedestalsonto said tacky areas; positioning said film relative to said integratedcircuit device such that said conductive pedestals are proximate to saidat least some of said plurality of contact pads on said integratedcircuit device; attaching said conductive pedestals to said at leastsome of said plurality of contact pads on said integrated circuitdevice; releasing said conductive pedestals from said film; positioningsaid integrated circuit device relative to said printed wiring boardsuch that said conductive pedestals are proximate to contact pads onsaid printed wiring board; attaching said conductive pedestals to saidcontact pads on said printed wiring board.
 2. The method at claim 1wherein said steps of attaching said conductive pedestals compriseattaching with solder.
 3. The method at claim 2 wherein said conductivepedestals comprise a material with a higher melting point than saidsolder.
 4. The method of claim 1 wherein said step of loading conductivepedestals comprises loading solder spheres.
 5. A method of attaching anintegrated circuit device to a printed wiring board comprising the stepsof: forming tacky areas on a film to correspond to at least some of aplurality of contact pads on said printed wiring board; loadingconductive pedestals onto said tacky areas; positioning said filmrelative to said printed wiring board such that said conductivepedestals are proximate to said at least some of said plurality ofcontact pads on said printed wiring board; attaching said conductivepedestals to said at least some of said plurality of contact pads onsaid printed wiring board; releasing said conductive pedestals from saidfilm; positioning said integrated circuit device relative to saidprinted wiring board such that said conductive pedestals are proximateto contact pads on said integrated circuit device; attaching saidconductive pedestals to said contact pads on said integrated circuitdevice.
 6. The method of claim 5 wherein said steps of attaching saidconductive pedestals comprise attaching with solder.
 7. The method ofclaim 6 wherein said conductive pedestals comprise a material with ahigher melting point than said solder.
 8. The method of claim 5 whereinsaid step of loading conductive pedestals comprises loading solderspheres.